Northeastern Section - 56th Annual Meeting - 2021

Paper No. 13-7
Presentation Time: 10:15 AM


THOMPSON, Margaret, Geosciences Department, Wellesley College, Wellesley, MA 02481, WHITE, C.E., Geoscience and Mines Branch, Nova Scotia Department of Energy and Mines, PO Box 698, Halifax, NS B3J2T9, Canada and BARR, S.M., Earth and Environmental Science, Acadia University, Wolfville, NS B4P2R6, Canada

Avalonia in SE New England is dominated by calc-alkaline plutons with ages between 620 and 604 Ma. These plutons, along with unconformably overlying units of the 597-593 Ma Lynn-Mattapan Volcanic Complex and 595-584 Ma Roxbury Conglomerate in the vicinity of Boston, MA, were formed during diachronous early Ediacaran arc magmatism characteristic of West Avalonian terranes extending to eastern Newfoundland. Plutonic ages in SE New England most closely resemble those in the Cobequid and Antigonish highlands of northern mainland Nova Scotia, and metasedimentary units including the < 912 Ma Westboro Formation (SE NE) and <945 Ma Gamble Brook Formation (NS) provide further links. Abundant Mesoproterozoic detrital zircon in these rocks has been attributed by different workers to source belts in both Amazonia and Baltica. The apparent absence of Tonian magmatic rocks in southeastern New England, has lately raised questions about the provenance of detrital zircon contributing to 745 and 647 Ma probability peaks in phyllite from Newport RI. This pattern has been interpreted to mean that southeasternmost New England represents a tectonic block of West African origin, but emerging details of the New England-Nova Scotia relationship suggest West Avalonian affinity of this area.

The 604 Ma Fall River Granite on the south side of the Beavertail shear zone--one possible location for the inferred tectonic boundary—is coeval with the newly dated Debert River and Frog Lake plutons in the Cobequid Bass River block. Both the Fall River Granite and 605 Ma andesite in the Jeffers block yield Sm/Nd ratios exceeding typical 0.17-0.20 crustal values. The 595 ± 5 Ma Dartmouth Pluton located approximately 15 km east of Newport overlaps 597-592 crystallization ages of the New Prospect Pluton, as well as volcanic tuffs and a porphyry in the Jeffers block. Several of the latter units yielded inherited zircon corresponding in age to the 647 Ma probability peak in the Newport phyllite. The 752 Ma Dalhousie Mountain Formation and 752-730 Ma plutons within the Mount Ephraim block record previously unrecognized volcanic arc activity consistent with the 745 Ma Newport population. Because the Newport minimum age is poorly constrained by a 595 ± 12 Ma Rb-Sr date, it is possible that the phyllite itself formed during some phase of Cobequid magmatism.